This is a common misconception. If someone says they want to bring you in as a cofounder, they almost always will think of you as a significant equity partner.
To be an equal you need equal equity but it is not that simple. Two friends holding 66% can dilute you, move your work into another company assgin a low value and cut you out. The many ways to get screwed is extremely high.
Like 40% down to 2%
If you are looking to be a cofounder, "founding engineer" is far from it.